In conventional systems automatic gain control and precise open loop gain tracking over a wide dynamic range is required for measuring time alignment error of the input signal. FIG. 1 discloses a typical conventional or prior art system for range tracking. RF (radio frequency) pulses are coupled into respective gates 10 and 20. The outputs from the respective gates are coupled through narrow band filters 12 and 22 respectively and into amplifiers 14 and 24 respectively. The output signals from the amplifiers are coupled into a phase detector 30 which provides an output signal indicative of the polarity of the error. The output voltage from amplifier 14 is further coupled through an AGC (automatic gain control) unit and fed back to both amplifiers 14 and 24 to provide gain control; while the output of amplifier 24 is coupled to provide the time alignment error signal output in conjunction with the polarity output of phase detector 30. The gain tracking between the two channels is required since the error is derived from an amplitude measurement. AGC is necessary to normalize the output error as a function of signal levels. Specifically, the absolute magnitude of the error signal from amplifier 24 varies both as a function of the time alignment error and as a function of the magnitude of the input signal. In order to make the error measurement meaningful, the fluctuation of the error signal as a function of input levels must be removed. The AGC accomplishes this normalization such that the output signal from amplifier 24 varies only as a function of the time alignment error.
RF pulses are fed into the two gates, gate 10 being a normal radio frequency gate, gate 20 being a split gate or early-late gate. The RF pulse gated through gate 10 is coupled through a narrow band filter to provide coherent integration of the RF pulses. The resulting output of the narrow band filter is a continuous wave signal coupled through amplifier 14. The RF pulse gated through split gate 20 results in a bipolar type output pulse whose crossover is lined up with the center of the normal gate pulse of gate 10. Thus the split gate provides a 180.degree. phase shift to the latter half of each gated signal. Therefore the RF output of the split gate which occurs during the first half of the split gate pulse is in phase with the RF input to gate 10, while the RF output occurring during the second half of the split gate pulse is 180.degree. out of phase with the RF input to gate 10. The magnitude of the central line of the frequency spectrum at the output of the split gate is a function of the time alignment error between the RF pulse and the split gate pulse. The phase of the signal with respect to the voltage output of filter 12 indicates the sense of the alignment error. The central-line of the frequency spectrum is extracted by the narrow band filter.